1. Field of the Invention
The present invention is a substantially continuous process for separating E and Z isomers of an alkene alcohol and derivatives thereof. An ion exchange medium which has been ion exchanged with silver and/or copper ions is contacted with a feed stream comprising a mixture of E and Z isomers of the alkene alcohol or derivative thereof as well as other components in a non-batch or substantially continuous mode, thereby enriching the resulting product stream in either the E or Z isomer.
2. Discussion of the Background
Synthetic methods for preparing unsaturated compounds which can exist in E and Z isomeric form usually provide a mixture of both isomers, yet quite often only one of these isomers is commercially valuable. It is therefore desirable to separate mixtures of E and Z isomers at commercially useful scales, and in a manner which is compatible with other processes.
Different methods have been used to separate mixtures of E and Z isomers. For example, E and Z isomers have been separated by fractional crystallization, precipitation, fractional distillation, solvent extraction, adsorption methods, extractive distillation and chemical transformation. These methods exploit differences in the crystallization, solubility, boiling point, volatility, reactivity and affinity and adsorption properties of the isomers. However, the difference between these properties in E and Z isomers is often relatively small, making such separations difficult and expensive
Processes such as fractional distillation and /or extractive distillation are energy intensive, and therefore can be expensive. Since the boiling points of E and Z isomers are often quite similar, distillation processes are often inefficient, or provide product streams in which the purity of the desired isomer is low. Furthermore certain isomers may have a very high boiling point or may decompose or unstable under distillation condition. Accordingly, adsorption processes such as chromatography have been receiving more attention recently because they have the potential to offer higher purity and higher efficiency (can also be operated under milder conditions) compared to other separation methods.
A variety of methods have been used to separate E and Z isomers. For example, U.S. Pat. No. 4,433,195 describes a process for separating non-polar cis- and trans-olefins (e.g., mixtures of Z and E butene; i.e. cis and trans 2-butene, respectively) using a silicalite adsorbent (silicalite is an organophilic form of silica). The separation may be carried out in either a single column, or using counter current moving bed or simulated moving bed flow systems.
Similarly, U.S. Pat. No. 3,600,453 also describes batch-type chromatographic separations of cis- and trans-olefins with a stationary phase of X or Y zeolites exchanged with various cations (i.e., copper, silver, gold, zinc, cadmium, and mercury). In this process, one of the isomers is selectively bound to the ion exchanged zeolite, and periodically desorbed from the stationary phase by interrupting the feed stream flowing into the column, and then stripping the selectively bound isomer from the column.
Various other workers have described methods of separating cis- and trans-unsaturated esters by conventional, batch-type chromatographic methods using a silver “loaded” stationary phase. For example, Morris, Journal of Lipid Research, vol. 7, (1966) pages 717–732, describes separating mixtures of cis- and trans-cyclodecenols and cis- and trans-unsaturated fatty acid esters on a column of silica gel impregnated with a stationery phase of aqueous silver nitrate. Houx et al., Journal of Chromatography, vol. 129, (1976) pages 456–459, describes separating unsaturated esters by HPLC using silver nitrate coated silica. Heath et al., Journal of Chromatographic Science, vol. 15 (1977), pages 10–13 describes the chromatographic separation of E and Z 9-tetradecenol acetate using silver nitrate coated silica. Lam et al., Journal of Chromatographic Science, vol. 15, (1977), pages 234–238 describe separating cis- and trans-p-bromophenacyl esters of unsaturated fatty acids on a silver loaded alumina silicate stationary phase or a silver ion exchanged ion exchange resin. Nikolova-Damyanova et al., Journal of Chromatography, 609, (1992), pages 133–140 describe the chromatographic separation of cis- and trans-unsaturated fatty esters using a silver nitrate coated silica stationary phase. Emken et al, Journal of the American Oil Chemist's Society, May 1964, vol. 41, no. 5, pages 388–390, describes the chromatographic separation of cis- and trans-fatty esters using a silver “saturated” ion exchange resin.
Thus, separations of E and Z isomers of unsaturated compounds having polar functional groups, e.g., alkene alcohols or esters, have only been carried out using conventional batch-type chromatographic methods. However, such chromatographic methods have insufficient productivity and purity for commercial use. Additionally, they consume more eluent and generate products with lower concentration. Since they are run in a “batch” mode, rather than a continuous or semi-continuous mode, such methods are also more expensive and complex to operate compared to a continuous process.